1. Field of the Invention
This invention relates to a method of and an apparatus for measuring the radius of curvature of a cornea, a lens or the like, and more particularly to a simple method of and a simple apparatus for measuring the directions of two principal meridians of an object to be examined including a toric surface and the radii of curvature in those directions.
2. Description of the Prior Art
As a general method of measuring a radius of curvature, there is known a method comprising projecting onto an object to be examined two object projecting light spots disposed at the opposite sides of the optical axis of an objective lens, obtaining the spacing between the reflected images of the light spots formed at predetermined positions in a measuring optical system including the objective lens, and measuring the radius of curvature of the object from this spacing. In this method, where the object is a spherical surface, the spacing between the reflected images directly corresponds to the radius of curvature of the object, but where the object includes a toric surface, the spacing between the reflected images does not directly correspond to the values corresponding to the radii of curvature in the directions of two principal meridians as the measured values. This is because, where the object includes a toric surface, the direction in which the reflected images are created is twisted by a predetermined amount in accordance with the projection directions of the light spots relative to the direction in which the reflected images are created where the object is a spherical surface (the direction passing through the two reflected images). In the method according to the prior art, attention is paid to the fact that the above-mentioned twist does not occur if the projection directions of the light spots are coincident with the directions of the principal meridians of the object, and the curvatures in the directions of the two principal meridians are obtained from the spacing between the reflected images created when said coincidence has occurred.
In a specific example using such method, if the standard line overlapping the two reflected images created when the object is, for example, a spherical surface is made observable from the viewfinder and the design is such that the two reflected images and the standard line overlap each other by the entire apparatus being rotated and the then spacing between the reflected images is optically or photoelectrically obtained and the then rotated position of the apparatus is obtained, then the radius of curvature and the amount of twist in the direction of one principal meridian will be obtained. The radius of curvature in the direction of the other principal meridian will be obtained if the apparatus is rotated by 90.degree. and the spacing between the two reflected images at that time (when of course the two reflected images and the standard line are overlapping each other) is obtained.
However, in the above-described measuring method according to the prior art, whether the object to be examined is a spherical surface or a toric surface cannot be discriminated by the measurement only in the direction of one meridian and accordingly, it is necessary to effect measurement in the directions of two meridians and this is time-consuming and, where the object to be examined is a toric surface, it is necessary to measure the spacing between the reflected images in the directions of two principal meridians and the direction of at least one principal meridian and thus, the measurement has been cumbersome.
In any case, the measuring method according to the prior art has involved a number of measuring procedures and has not been preferred as a curvature-radius measuring method when simplicity is desired.